Email sent from libmail.php does not reaches outlook mail

In my website, i send an automated email using libmail.php for new sign-ups. The user puts in his email address in my website and he gets an email to activate his account. This usually works for gmail. However when a user signs up with outlook i have noticed no email gets through to the user. In my codes, i actually have done a bcc to another gmail account from which iam sure that the activation email was indeed sent to the user, but it doesn't reach outlook email. Is there anything that i can do about this? Thank You.